color-selection: Take hex codes that don't start with "#"

Previously if a user input a valid hex code, but left out
"#", `gdk_rgba_parse ()` would fail.

This commit ensures that we prepend "#" before parsing, so
that codes without it are still handled properly.

Closes #109
1 job for master in 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #487589
flatpak

00:00:47